grouping targets in the target list

[fbstj]

I came across #2170 and the screenshot inspired me to ask if it makes sense to group the targets by platform tier

[Nemo157]

AFAIK there's no tier metadata published with the targets, https://github.com/rust-lang/rust/blob/master/src/doc/rustc/src/platform-support.md is the only place I know we could get this info.

It might also be that rustc's tiers don't match the crate's tiers, a lot of the time the only reason crates build for more than a single target is that they don't override the default target list. If we're going to provide grouping it might be best set by the crate itself

[[package.metadata.docs.rs.target-groups]]
label = "Fully tested"
targets = [ ... ]
[[package.metadata.docs.rs.target-groups]]
label = "Build only"
targets = [ ... ]